1. Field of the Invention
The present invention relates to an optical proximity correction method. More particularly, the present invention relates to an optical proximity correction method used on highly integrated devices with small critical dimension.
2. Description of the Related Art
As developments in circuit integration continue to proliferate, design trends will compel further reductions in device size and higher levels of device integration.
During manufacture, the photolithography process is one of the most decisive steps in determining the performance of an IC. Following a gradual increase in the level of circuit integration and a concomitant reduction in device size, problems have emerged related to pattern transfer deviation. These problems occur during photolithography.
For example, when a masking pattern is transferred to a wafer using photolithographic etching, the angles on the pattern become less sharp, the tail of the pattern shrinks, and line-width increases or decreases. This is what is known as the Optical Proximity Effect (OPE).
With larger devices or those that are less integrated, this deviation does not have an extremely adverse effect. However, this deviation has a critical effect on the performance of highly integrated devices. For example, on highly integrated ICs, distance between devices is small. Thus, when the line width of the pattern that has been transferred to the wafer expands, it is possible for patterns to partially overlap causing a short circuit. Enhancing the performance of ICs, that is to say, the improvements that can be gained corresponding to a reduction in device size, will gradually be constrained by the increasing lack of fidelity in the lithographic etching process.
The primary factors related to the formation of OPE are optical factors. These factors can include interference caused by light piercing two different patterns on the photomask, the reflection of light caused by an uneven substrate, as well as other factors related to manufacture of the photomask such as baking time, baking temperature and development. When the critical dimension decreases as the size of the pattern decreases, deviation caused by OPE during transfer will be even greater.
An optical proximity correction (OPC) procedure is often performed to prevent Critical Dimension Variation during the transfer of the photomask pattern. That is to say, a data computer and commercial software are used to measure and correct the original pattern exposed on the wafer of the semiconductor substrate. The resulting pattern is saved in a computer file. This pattern is then fabricated on the photomask. Thus, the patterns created as light penetrates the mask and is projected on the semiconductor substrate will be nearly the same as the original.
Optical Proximity Correction can be divided into rule-based Optical Proximity Correction and model-based Optical Proximity Correction. Rule-based OPC involves comparing the width of the original pattern that is to be transferred and the relative density of features to the correction standard saved in the database to perform correction. Model-based OPC primarily uses a simulator to conduct a series of complex corrective calculations according to the exposure result.
Rule-based OPC adds serifs to the comers or hammerheads to the edges of the original pattern. The width of the features of the original pattern to be transferred and the density of features on the pattern are compared to a reference table, established according to an obtained corrective standard. Based on this comparison assist features are added to perform the correction.
As shown in FIG. 1, a main pattern 100 to be transferred is provided, including rectangular region 100a, long narrow region 100b and the connected portions 100c and 100d. Serifs 110 are added to the main pattern 100. For example, serifs 110 are added to each of the comers of rectangular region 100a and long narrow region 100b. Depending on the circumstance (based on width and density), the line width of the original pattern is increased or decreased or assist features such as scattering bars 120 are added around the pattern.
Scattering bars are among the most commonly used assist features in the photolithography process. Scattering bars are added by forming bar-shaped openings on the periphery of the pattern on the photomask. When the pattern is exposed, light waves pass through the scattering bars so that the proximity effect on the original pattern is eliminated. Scattering bars can adjust the duty ratio between line width to achieve the best resolution.
The addition of serifs or hammerheads can reduce rounding of the corners and edges of the transfer pattern. The addition of assist features increases the pattern""s contrast to achieve the best resolution and increase the process window.
In the model-based OPC method a test pattern is exposed. A correlation is made between the exposed result and the original pattern to be transferred. The degree of deviation between the two is used to decide whether or not to perform optical proximity correction OPC. The simulator is then used to calculate and determine the degree of correction, based on the degree of deviation. As shown in FIG. 2, a simulator is used to calculate and correct the exposure of the main pattern to be transferred in FIG. 1 result to achieve corrected pattern 200. Corrected pattern 200 includes rectangular area 200a, long narrow area 200b, and the connected areas 200c and 200d. The simulator is used to perform comprehensive calculations on the two dimensional pattern based on the specific reference indexes of the coordinate system. According to the calculations, a series of features are added. The shape of the original pattern is thus changed to adjust for the error that may occur during pattern transfer. Moreover, the process of calculation is repeated several times. Thus, the accuracy of the model-based OPC method is very high. However, the assist features of this method do not include scattering bars. Consequently, the processing window of this method cannot be increased.
The primary object of this method is to provide an optical correction method that uses a model-based OPC approach suited for use with assist features, such as scattering bars, which is able to increase the processing window and maintain accuracy.
Another object of this invention is to provide an optical proximity correction method that can be used to correct the photomask design of highly integrated devices with low critical dimension.
As embodied and broadly described herein, the invention first provides a layout pattern to be transferred. Assist features, such as scattering bars, are added to the layout pattern where the critical dimension is smaller than the wavelength of the exposed light source. Using a model-based optical proximity correction method, the simulator is used to perform calculations on the entire original two-dimensional pattern. The scattering bars are not included in this calculation and correction, however. A photomask is formed according to the calculation result. Exposure and other steps of the photolithography process are conducted.
According to the object of this invention, the use of optical proximity correction and the assist features added to the main pattern to be transferred can increase the pattern""s contrast achieving the highest resolution and can increase the processing window. The calculations and corrections using the simulator to perform model-based OPC also enhance accuracy.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.